USGS Open-File Report 94-023
GCM Simulations Of The Pliocene Climate: Feedbacks, Ocean Transports, And CO2
Estimates of sea surface temperatures (SSTs), based on marine microfauna,
reveal the existence of a middle Pliocene warm period between about 3.15
and 2.85 million years ago (Dowsett et al., 1992). Terrestrial pollen
records, although not as well dated, also show evidence for a warmer
climate at about this same time in the Pliocene and, further, indicate
that continental moisture levels varied significantly from the present
day. The cause of the altered climate is not known with certainty, but
sensitivity experiments, conducted using the Goddard Institute for Space
Studies General Circulation Model (GISS GCM), have indicated that warmer
climates, such as those of the Pliocene, can be simulated by using
increased ocean heat transports (Rind and Chandler, 1991) [figure 1].
Dowsett et al. (1992) suggested that this might be the case for the
Pliocene, based on the distribution of North Atlantic SSTs.
- Mark Chandler
- NASA/GISS, New York, New York 10025
One test of this hypothesis is to supply Pliocene SSTs, together with an
estimate of the terrestrial vegetation coverage, as boundary conditions
in a GCM simulation, and to examine the temperature feedbacks and moisture
changes that result. Consistency between the palynological estimates of
climate and the simulation results provide one level of validation for the
GCM; the GCM then provides a method for investigating the atmospheric
processes involved in maintaining the warmer climate.
- Figure 1. Pliocene and modern annual ocean heat transports
for the Atlantic Ocean
- This figure is available as a
Using the GISS GCM together with PRISM Northern Hemisphere ocean surface
and vegetation boundary conditions (Chandler et al., submitted; Dowsett et
al., submitted) we found both consistencies and inconsistencies between
model and data-generated paleoclimate estimates. Generally, temperature
estimates show the greatest consistency, with both model and data
indicating significantly warmer temperatures at high latitudes and
diminished warming nearer to the equator. The GCM yields temperature
increases up to 10°C along the Arctic coasts and shows greatest warming in
the winter. Although the original temperature increase is driven by
warmer SSTs, much of the continental interior warming is generated by an
ice-albedo feedback, as reduced snow cover in the warmer climate increases
the absorption of solar radiation at the surface during winter months
[figure 2]. Further warming at high latitudes comes from the increased
levels of atmospheric water vapor (a greenhouse gas) that are a result of
the warm, ice-free ocean conditions. Despite the generally warmer
climatic conditions, some areas show overall cooling. Notably, East
Africa cools by 2 to 3° C due to increased low-level cloud cover which
reflects large amounts of incoming solar radiation back to space. This
result is consistent with the single palynological record that exists for
Model-data moisture estimates show far less consistency than do the
temperature estimates, not a surprising result given the complexities
involved in modeling hydrologic processes using coarse-grid numerical
models like the GISS GCM. The most common discordance seems to be an
underestimation of the increased wetness suggested by pollen records at
several localities throughout the Northern Hemisphere. The model's simple
ground hydrology responds to the warmer summer ground temperatures by
drying out while the diminished intensity of the atmospheric circulation
(a result of reduced latitudinal temperature gradients) decreases the
amount of moisture advected from over the oceans to the continents. In
the Arctic, where modern tundra environments were replaced by Pliocene
boreal forests, the altered boundary conditions required that wetter soil
moisture conditions be specified. The Pliocene Arctic soils remained
wetter than the present day, indicating that the specified wet conditions
were in equilibrium with the simulated climate.
- Figure 2. Water vapor, cloud coverage, and ground albedo in
the northern hemisphere as a function of latitude in the GISS Pliocene GCM
- This figure is available as a
In addition to the above experiment, several simulations were conducted
using increased levels of atmospheric carbon dioxide; higher CO2 amounts
have also been proposed as a potential cause of the warmer Pliocene
climates (Crowley, 1991). Rind and Chandler, (1991) pointed out that SST
patterns such as the one seen in the Pliocene are inconsistent with CO2
generated warming, however, it is possible that some combination of CO2
increase and ocean heat transport change could have resulted in the warmer
Pliocene surface temperatures. Figure 3 shows the various levels of ocean
heat transport required to generate the PRISM SSTs given various
atmospheric CO2 increases. The graph indicates that with modern ocean
heat transports (0% increase) CO2 levels must have been at least 1400 ppm
(4.5 times the modern value) in order to generate the global warming of the
Pliocene. So far, estimates based on carbon isotope measurements by Raymo
and Rau (1992) suggest that Pliocene CO2 levels were, at most, 100 ppm
greater than today.
- Figure 3. Levels of ocean heat transport required to
generate PRISM sea-surface temperatures at varying level of atmospheric
- This figure is available as a
- Chandler, M. A., Rind, D., and Thompson, R. S., submitted, A simulation of the middle Pliocene climate using the GISS GCM and PRISM Northern Hemisphere Boundary Conditions: Global and Planetary Change Section of Palaeogeography, Palaeoclimatology, Palaeoecology.
- Crowley, T. J., 1991, Modeling Pliocene Warmth: Quaternary Science Reviews, v. 10, p. 275-282.
- Dowsett, H. J., Cronin, T. M., Poore, R. Z., Thompson, R. S., Whatley, R. C., and Wood, A. M., 1992, Micropaleontological evidence for increased meridional heat transport in the North Atlantic Ocean during the Pliocene: Science, v. 258, p. 1133-1135.
- Dowsett, H. J., Thompson, R. S., Barron, J. A., Cronin, T. M., Ishman, S. E., Poore, R. Z., Willard, D. A., and Holtz, T. R., Jr., submitted, Paleoclimatic reconstructions of a warmer Earth: PRISM Middle Pliocene Northern Hemisphere Synthesis: Global and Planetary Change Section of Palaeogeography, Palaeoclimatology, Palaeoecology.
- Raymo, M. E. and Rau, G., 1992. Plio-Pleistocene atmospheric CO2 levels inferred from POM ¶13C at DSDP Site 607 (Abstract). Eos, Transactions of the American Geophysical Union, 1992 Fall Meeting Supplement, no. 73, p. 95.
- Rind, D., and Chandler, M. A., 1991, Increased ocean heat transports and warmer climate: Journal of Geophysical Research, v. 96, p. 7437-7461.
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